In the past, mobile communication operators of one country or region only deploy access networks of one standard. Under this network condition, a user obtains services through a single-mode terminal. Taking a worldwide view, networks of different standards coexist.
With the selections of the operators in deploying networks as well as the mergers and obtainings between the operators, one operator may operate networks of multiple standards at the same time. Moreover, the growing demand of the user for mobile applications greatly promotes the rapid development of access technologies, so that many new access technologies, for example, Wireless Fidelity (WiFi), Worldwide Interoperability for Microwave Access (WiMAX), etc., come forth continuously. Therefore, providing the users with a seamless service transfer to ensure service continuity under heterogeneous access technologies has become an urgent requirement for the operators to satisfy the users' requirements and enhance their own competitiveness. Currently, a same set of core networks to support different access technologies and meanwhile support a terminal of different access modes (i.e. a multi-mode terminal, and a CSI terminal and a VCC terminal are both applications of the multi-mode terminal) is put forward to satisfy this requirement.
The combination of circuit-switched (CS) and Internet protocol (IP) multimedia system (IMS) services (CSI, combining CS bearers with IMS) is proposed by the 3rd Generation Project Partnership (3GPP), for combining advantages of a CS domain and an IMS domain by enhancing terminal capabilities and providing the users with multimedia service experience in conjunction with a network entity (CSI-AS) in the circumstance that an access network side supporting packet-switched (PS) domain can not bear real-time media.
In the above CSI solution, in terms of access network capabilities, a global system for mobile communications (GSM) network is required to support the dual transfer mode (DTM) technology, or a wide-band code division multiple access (WCDMA) network is required to support multi radio access bearer (multi RAB) technology so as to provide the CSI. A non-supporting access network can only use bearers of one type at the same time.
In the above CSI solution, in terms of terminal capabilities, the terminal is required to have the capability of providing a session association on networks of different access types to the user, and meanwhile the associated sessions may contain different media components. For example, a session association between an IMS voice call on a wireless local area network (WLAN) bearer and an IMS text of a PS bearer is provided to the user.
In the above CSI, the CSI-application server (CSI-AS) has the following functions.
1. Selection of whether to combine sessions established on different access networks initiated by the CSI terminal (caller) according to network policies.
2. Splitting of an IMS session to different access networks registered by the CSI terminal (as a callee) for connection after receiving the multimedia IMS session.
3. Generating of charging information related to the CSI.
4. Supplementary service processing related to the CSI.
Taking the splitting of the multimedia IMS session by the CSI-AS as an example, referring to FIG. 1, after receiving the multimedia IMS session, the CSI-AS splits the IMS session into a CS call and an IMS call, i.e. bears a real-time media component, for example, audio, video, etc., on a CS network, and bears a non-real-time media component, for example, text, etc. on a packet-based network, and may also bear the video in the PS domain if it is bearable for the user. Meanwhile, in order to ensure the service experience, the CSI requires that the real-time and non-real-time media components must be terminated to the same terminal of an opposition end user.
The voice call continuity (VCC) is an application provided in a home IMS network of the user, which enables a bi-directional transfer of a voice call of the user between the CS domain and the IMS network. The integrated IMS architecture makes it possible to provide a popular GSM voice call under the WLAN coverage. If the seamless voice call service is implemented between the CS domain and an IP connectivity access network (IP-CAN), not only the load of GSM/universal mobile telecommunications system (UMTS) is reduced, but also the gain of the operator is increased. In addition, the wired operator providing the Voice over IP (VoIP) service may also benefit from the integrated services provided by the 3GPP IMS architecture.
FIG. 2 shows an implementation architecture of 3GPP VCC. A set of functional entities is newly added in the IMS domain and the CS domain. Those functional entities are a route redirection entity, a CS domain adaptation entity, a domain selection control entity, and a domain transfer control entity.
1. The route redirection entity (the customized applications for mobile network enhanced logic (CAMEL) app in FIG. 2) is responsible for redirecting a CS domain call to the IMS domain to perform a call anchoring control. In general circumstances, the route redirection entity is co-located with the gsm service control function (SCF) in FIG. 2 and embodied as a service control point (SCP) in the CS domain.
2. The CS domain adaptation entity (the CS adaptation function (CSAF) in FIG. 2) is responsible for receiving the CS domain call redirected to the IMS domain and converting the call into an IMS domain call according to stored information (possibly information obtaining trough interacting with the CAMEL App).
3. The domain selection control entity (the domain selection function (DSF) in FIG. 2) is responsible for making a decision according to various policies such as a registration status and a call status of the user in the IMS domain, and controlling the call to be routed to a selected connection domain.
4. The domain transfer control entity (the domain transfer function (DTF) in FIG. 2) is responsible for anchoring the call in the IMS domain and controlling a transfer when transfer happens.
The above four functional entities are collectively referred as a VCC service control entity.
Based on the above VCC service control entity, when the VCC terminal is conducting a voice session of an activity, a domain transfer may be initiated. In order to perform the domain transfer, a call initiated or accepted by the VCC terminal must be anchored to a DTF in a home IMS network of the VCC terminal. The DTF is an AS within a 3rd party call control (3PCC) function. In the VCC solution, a session control leg between the DTF and the VCC terminal is called an access leg, and the session control leg between the DTF and a remote user is called a remote leg. The transfer is replacing an old access leg by a new access leg. In general circumstances, the VCC terminal is able to sense the strength of radio signals of access networks more accurately than a core network, and thus domain transfer processes with high requirements for a delay are initiated from the terminal towards the network. When the VCC terminal of the user detects radio signals and other factors and determines that it needs to transfer from a source network to a destination network, the VCC terminal calls a special number in the destination network. A call request for the special number may be triggered to the DTF. As the original call has already been anchored at the DTF, the DTF associates the old and new calls according to a user ID. A media is renegotiated with the remote user terminal of the original call according to the media in the newly established call, and a media flow corresponding to the transferred call is redirected from a port of a multi-mode terminal in the transferred-out access network to a corresponding port in the transferred-in access network at the same time during the media renegotiation process. As the media negotiation is completed, the new call is established successfully. Then the call in the transferred-in access network is released by the DTF or the VCC user equipment (UE). Thus the voice call of the user is transferred to the destination network. During the transfer, the voice call of the user remains uninterrupted, thereby improving the user's service experience.
FIG. 3 is a flow chart of transferring a call from a CS domain to an IMS domain in the VCC.
0-1. A VCC UE determines that the call needs to be transferred to the IMS domain according to the wireless environment and calls a special number i.e. VCC domain transfer URI (VDI) in the IMS domain to initiate a domain transfer. The call request is forwarded from a proxy call session control function (P-CSCF) to a serving call session control function (S-CSCF) in the home IMS network of the VCC user for processing.
2. The S-CSCF triggers the call to the DTF for processing according to initial Filter Criteria (iFC) of the caller.
3. The DTF determines that the call is a domain transfer request according to the VDI in an INVITE, and finds the anchored session according to caller information.
4-8. The DTF acts as an agent of the user to renegotiate a session description protocol (SDP) by using media information in the transfer request with the remote user.
9. After the session of the destination network is established, the DTF releases call resources in the access part of the CS domain of the VCC user.
Based on the above contents related to the VCC, the following scenarios are considered.
In a first scenario, the multi-mode terminal supports UMTS terrestrial radio access network (UTRAN) CS access, UTRAN PS access and WLAN access, and can register with the IMS network through the PS and the WLAN. The user has the VCC service function. The user is registered in the CS domain and processes a call, and not registered in other domains. When the user moves to the cross-coverage covered by the UTRAN and WLAN (the CS session can be reserved due to the cross-coverage), the user initiates an IMS registration on a WLAN bearer and receives a non-real-time media session sent from the called side. If the user continues moving to a region only covered by the WLAN, a real-time media component borne on the CS network needs to be transferred to the WLAN bearer. Further, as only one session exists on the remote leg, the real-time media component should be transferred to a session where the non-real-time media component resides. A similar situation exists for the non-real-time media component.
In the above first scenario, if the user moves out of the WLAN coverage area after receiving the non-real-time media session sent from the called side, the multi-mode terminal activates the access of the PS domain under the UTRAN and registers with the IMS through the PS. Then, a transfer is initiated to transfer the non-real-time media session of the WLAN bearer to the PS bearer. A similar scenario exists for the real-time media session.
In a second scenario, when the multi-mode terminal only registers with the IMS accessed by the WLAN, a multimedia session is performed with the opposition end user. Then, the user moves to an edge region covered by the WLAN, and the registration in the CS domain is activated as the multi-mode terminal detects that the signal in the CS domain is better. A transfer of the real-time media in the multimedia call to the CS domain bearer is initiated.
The following media flow transfer modes that possibly occur may be derived from the above scenarios.
1. The real-time or non-real-time media component in the multimedia session is transferred to other bearers.
2. The real-time or non-real-time media session constituting associated sessions is transferred to other bearers.
3. The real-time or non-real-time media session constituting associated sessions is transferred to an existing session on other IP-CANs.
In the process of the invention, the inventor finds that the existing VCC technology only solves the problem of transfer of the voice session between bearers of different access modes, and thus can only implement the voice session continuity, but does not support the aforementioned media flow transfer modes that may possibly occur. That is, the existing VCC technology cannot solve the problem of transferring part of the media flow in the multimedia session on the multi-mode terminal between bearers of different access modes.